I would say more than 2 is easily doable but you're right that express is needed. We get 'Frequent Fear' tickets which gives us multiple dates every week it runs, we don't upgrade to express as we don't need to. However we visit 3 to 4 nights each year, on the 3rd we complete all houses and the 4th we re-visit ones we've liked most.
If you went one night only you'd struggle. It opens officially at 6:30pm, however letting everyone in at 6:10pm isn't uncommon. If you'd arrived about 4:30-5pm like we do, you'd be at the front of the queue when they open. As you enter you can hit certain houses with no queues, after the first you should easily find a second with a 15 min queue straight after. Then you could probably do a third with not more than a 30 minute wait. After that they become rather long waits.
Another thing to consider is some houses can have 45 minute waits (or longer) right after it opens. This is because day guests who've bought a HHN ticket too can enjoy usually 2 houses that open about 5:30pm as they're allowed to stay on in the park and have a head start. If you're there from opening time to closing for 1 night only, you possibly could enter every house without express but it's highly unlikely unless you're both very smart and lucky.
